Voltage multiplying (e.g., voltage doubling), circuits are commonly used in low power portable communication devices (e.g., selective call receivers), to raise an incoming battery voltage (nominal 1.5 volts) up to an operating voltage of approximately 3.0 volts for a CMOS microcomputer and associated logic circuits. Since CMOS current requirements are typically very low (unit milliamps or less), the battery and voltage multiplying circuits can usually supply the higher voltage at the rated current load.
A typical topology of conventional voltage multiplying circuits includes switching circuits to selectively transfer an input voltage potential to an intermediate energy storage device, such as a capacitor. Additional switching circuits serially couple the intermediate voltage potential of the first energy storage device and the input voltage potential to a second "output" energy storage device. The rate at which the output voltage potential approaches twice that of the input voltage potential typically depends on the switching speed of the switching circuits, the capacity of the energy storage devices (i.e., the size of the capacitors), and the current handling capability of the switching circuits. Normally, the switching circuits are controlled by control circuits that receive timing signals from a timing circuit; all powered from the input voltage source. Although, in some conventional designs a "bootstrap" control circuit is powered from the output voltage to enhance the current handling capability of the switching circuits, as is subsequently described herein.
Referring to FIG. 1, a conventional configuration for a voltage multiplying (doubling) circuit is shown. An incoming voltage (Vin) at an input 10 is selectively transferred to an energy storage device, such as provided by capacitor 22, using switching devices (12 and 14). The switching devices (12 and 14) typically comprise MOS transistors and respond to a control signal 16, which is generated by a control circuit 18. In similar fashion, switching devices (24 and 26) respond to a control signal 28 from control circuit 20. Control circuits 18 and 20 are configured as sequential inverters (or optionally as non-overlapping timing generators), such that the control signals 16 and 28 are always the complement of each other. Therefore, when switching devices (12 and 14) are "closed" switching devices (24 and 26) are "open", and vice versa. In the latter case, the intermediate voltage potential across the capacitor 22 is selectively serially coupled to, or superimposed on top of (added to), the input voltage potential (Vin), and presented to a second "output" energy storage device, such as provided by a capacitor 30. Lastly, timing signals from a timing circuit (not shown) are routed to the control circuits (18, 20, 32, and 34) via an input 40 to control the switching speed of the switching circuits (12, 14, 24, and 26).
In this configuration, when the output voltage potential (Vout) increases above a minimum operational voltage threshold for control circuits (32 and 34), control signals (16' and 28') supplement the control signals (16 and 28). The combined control signals (16, 16', 28, and 28') drive the switching devices (12, 14, 24, and 26) harder (more fully conductive) and improve their current handling capability. Therefore, as the "output" voltage potential (Vout) across capacitor 30 increases above a minimum threshold, the increased current handling capability of the switching devices (12, 14, 24, and 26) increases the rate at which the output voltage potential (Vout) approaches twice that of the input voltage potential (Vin). Hence, the efficiency of the voltage multiplying circuits is improved when control circuits (32 and 34) begin to provide additional control signals (16' and 28').
Besides increasing the rate at which the "output" voltage potential (Vout) approaches twice that of the input voltage potential (Vin), a second important requirement demands the overall lower voltage operation for voltage multiplying circuits. Conventional designs require typically 1.5 volts (and higher) for the input voltage (Vin). Below that range, the control circuits (18, 20, 32, and 34) as well as the switching devices (12, 14, 24, and 26) normally will not operate efficiently, if at all. Specifically, the control circuits (18 and 20) may operate, however the switch resistance may be too high for startup of normal multiplier operation. With the ever increasing demand for longer battery life, which is directly affected by the operational range of the voltage multiplying (doubling) circuits, it is regrettable that no known configuration exists to improve the minimum operational voltage threshold substantially below the conventional range.